1. Field of the Invention
This invention relates to electronic article surveillance apparatus and more particularly it concerns novel surveillance responders having response characteristics which are substantially independent of the material of the object to which the responders are attached.
2. Description of the Prior Art
U.S. Pat. Nos. 4,700,179 and No. 4,471,344 disclose anti-theft article surveillance systems wherein an article to be protected against the unauthorized taking from a surveillance area has a transponder mounted thereon and wherein transmitter and receiver antennas are arranged at an exit from the area. The transmitter antenna generates electromagnetic interrogation waves at the exit; and if an article on which a transponder is mounted is carried through the exit, the transponder disturbs the interrogation waves in a predetermined manner, for example, by producing waves which are harmonics of the interrogation waves. The receiver antenna is connected to a receiver system which responds to the predetermined disturbances to produce an alarm.
The above mentioned patents describe interrogation waves whose frequency is in the range of 915 megahertz and transponder produced disturbances in the range of 1830 megahertz. Other electronic article surveillance systems operate at frequencies which are much lower; but an advantage of using these high frequencies is that the transponders mounted on the protected articles can be made quite small and the system can be used with wide exit passageways. A disadvantage of article surveillance systems which use high frequency waves has been that the presence of certain materials, such as metal, will interfere with the performance of the transponder. Therefore, systems using such high frequencies have not been suitable for protecting articles made of metal.